JTextPaneでキーワードのSyntaxHighlight

// This code is taken from: SyntaxDocument.java, MultiSyntaxDocument.java
// Fast styled JTextPane editor | Oracle Community
// @author camickr
// @author David Underhill
// https://community.oracle.com/thread/2105230
// modified by aterai aterai@outlook.com
class SimpleSyntaxDocument extends DefaultStyledDocument {
  private final Style normal;
  private static final String OPERANDS = ".,";
  public SimpleSyntaxDocument() {
    super();
    Style def = StyleContext.getDefaultStyleContext().getStyle(
        StyleContext.DEFAULT_STYLE);
    normal = addStyle("normal", def);
    StyleConstants.setForeground(normal, Color.BLACK);
    StyleConstants.setForeground(addStyle("red",   normal), Color.RED);
    StyleConstants.setForeground(addStyle("green", normal), Color.GREEN);
    StyleConstants.setForeground(addStyle("blue",  normal), Color.BLUE);
  }

  @Override public void insertString(int offset, String str, AttributeSet a)
        throws BadLocationException {
    super.insertString(offset, str, a);
    processChangedLines(offset, str.length());
  }

  @Override public void remove(int offset, int length)
        throws BadLocationException {
    super.remove(offset, length);
    processChangedLines(offset, 0);
  }

  private void processChangedLines(int offset, int length)
        throws BadLocationException {
    Element root = getDefaultRootElement();
    int startLine = root.getElementIndex(offset);
    int endLine = root.getElementIndex(offset + length);
    for (int line = startLine; line <= endLine; line++) {
      applyHighlighting(line);
    }
  }

  private void applyHighlighting(int line) throws BadLocationException {
    Element root = getDefaultRootElement();
    int startOffset   = root.getElement(line).getStartOffset();
    int endOffset     = root.getElement(line).getEndOffset() - 1;
    int lineLength    = endOffset - startOffset;
    int contentLength = getLength();
    if (endOffset >= contentLength) {
      endOffset = contentLength - 1;
    }
    setCharacterAttributes(startOffset, lineLength, normal, true);
    checkForTokens(startOffset, endOffset);
  }

  private void checkForTokens(int startOffset, int endOffset)
        throws BadLocationException {
    int index = startOffset;
    while (index <= endOffset) {
      while (isDelimiter(getText(index, 1))) {
        if (index < endOffset) {
          index++;
        } else {
          return;
        }
      }
      index = getOtherToken(index, endOffset);
    }
  }

  private int getOtherToken(int startOffset, int endOffset)
        throws BadLocationException {
    int endOfToken = startOffset + 1;
    while (endOfToken <= endOffset) {
      if (isDelimiter(getText(endOfToken, 1))) {
        break;
      }
      endOfToken++;
    }
    String token = getText(startOffset, endOfToken - startOffset);
    Style s = getStyle(token);
    if (s != null) {
      setCharacterAttributes(startOffset, endOfToken - startOffset, s, false);
    }
    return endOfToken + 1;
  }

  protected boolean isDelimiter(String character) {
    return Character.isWhitespace(
        character.charAt(0)) || OPERANDS.indexOf(character) != -1;
  }
}